Practical investigations
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I'm doing an investigation relating to the degradation of wines and the effect on total acidity content for my coursework. To find the total acidity I've titrated 0.1 M NaOH with 20 cm3 of my wine samples, but after reading through your tutorial I'm a little confused about the calculation involved. I know that it is carried out in terms of representative acid but I am unsure as to whether the acid chosen (e.g tartaric) actually has any involvement in the calculation other than to work out the molar ratio, or is it a simple case of working out the moles of NaOH used and then dividing that by the volume of wine used to work out acid concentration? Thanks.
Igloo replies ...
You cannot carry out a conventional calculation from your titration results, since the wine contains a mixture of acids. Some of these are monoprotic (monobasic), e.g. ethanoic acid, whereas others will be diprotic (dibasic), e.g. tartaric acid, and a few are triprotic (tribasic), e.g. citric acid. Therefore you cannot calculate the total number of moles of acid present, since each type requires a different number of moles of alkali for neutralisation.
One mole of ethanoic acid reacts with one mole of NaOH
One mole of tartaric acid reacts with two moles of NaOH
One mole of citric acid reacts with three moles of NaOH
If you knew the proportion of these three different types of acid present then it might be possible to carry out a more accurate calculation, but you don’t, so you have to compromise and choose one acid which you believe to be in greatest abundance, perhaps tartaric acid. From this point onwards you now have to assume that ALL the acid in the wine is tartaric acid, and carry out your calculations accordingly. The titre will give you the moles of NaOH used. This needs to be divided by two to give the moles of tartaric acid present, and then you will need to scale up to find the total number of moles of tartaric acid present in 1 dm3 of wine.
Finally when stating your results you need to point out that the calculations have been carried out on the basis that tartaric acid represents all the acids present, and that in the circumstances there is no other way to proceed.
If tartaric acid happens to account for virtually all the acids present, then the calculation is going to give a realistic view of the situation. As it is, I doubt very much whether this is the case, so you’re stuck with a somewhat unsatisfactory resolution of the problem.
Incidentally since your investigation is concerned with wine degradation, it might be more appropriate to choose ethanoic acid as your representative acid, since this acid is the predominant one produced during the bacterial oxidation of ethanol. I would imagine that the concentration of acids such as tartaric would remain fairly constant during the degradation process. These are only my ideas – you must decide what use to make of them.
I hope that you understand things better now. Good luck.
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updated: 17 April 2007
